1. Field of the Invention
The present invention relates to a method of manufacturing a silicon substrate (referred to as an SOI (silicon on insulator) substrate throughout the specification) comprising a buried silicon oxide layer immediately under a silicon thin film serving as an active layer.
2. Description of the Background Art
When forming a device on the SOI substrate having the buried silicon oxide layer therein as described above, it is possible to reliably isolate the device with respect to the substrate. Therefore, a leakage current between elements is reduced and a device having excellent drivability (driving current, response speed etc.) can be formed. Further, an element isolation region such as a trench may not be deeply formed and can be inhibited from transverse spreading, to attain further refinement. Hence, the SOI substrate is applied to a high-frequency device used in the GHz band, a high-speed microprocessor or a low power consumption element, for example.
Such an SOI substrate can be manufactured by various methods such as an SOS (silicon on sapphire) method or an SIMOX (separation by implanted oxygen) method. Attention is now drawn to a bonding method for manufacturing an SOI substrate by bonding a bond wafer having a buried silicon oxide layer part and a base wafer serving as a support substrate to each other.
A conventional method of manufacturing an SOI substrate employing the bonding method is described with reference to FIG. 26. First, a bond wafer 1 consisting of a single crystal of silicon is dipped in an ionization solution. An electric field is applied between the bond wafer 1 and the ionization solution for ionizing silicon atoms present on the main surfaces of the bond wafer 1 and dissolving the same in the ionization solution (performing the so-called anodization). At this time, dissolution heterogeneously progresses on one of the main surfaces of the bond wafer 1 and a porous silicon layer (a silicon layer having numerous small grooves or depressions distributed in the crystal) 15 is formed on this main surface.
Then, a silicon single-crystalline layer 4 is formed on a surface of the porous silicon layer 15 by epitaxy. The overall surface of the bond wafer 1 is oxidized for forming a silicon oxide layer 5. Then, a base wafer 2 consisting of a single crystal of silicon is bonded to the main surface of the bond wafer 1 formed with the porous silicon layer 15. The bond wafer 1 and the base wafer 2 bonded to each other are heated to a temperature of at least 900xc2x0 C., for example, for reinforcing the degree of adhesion therebetween.
The bond wafer 1 is removed by polishing the main surface opposite from that formed with the porous silicon layer 15 serving as a stopper, and thereafter the porous silicon layer 15 is removed by dipping the base wafer 2, which is in close contact with the multilayer structure of the porous silicon layer 15, the silicon single-crystalline layer 4 and the silicon oxide layer 5, in a mixed solution of a hydrofluoric acid solution and aqueous hydrogen peroxide.
Thus, an SOI substrate having the silicon oxide layer 5 as a buried silicon oxide layer is obtained.
As hereinabove described, the porous silicon layer 15 is employed as a stopper when removing the bond wafer 1 in the conventional bonding method. This is because the porous silicon layer 15 has selectivity for single-crystalline silicon in polishing due to the coarse crystal state thereof.
However, the crystal state of the porous silicon layer 15 is disadvantageously irregular. When formed on the surface of the porous silicon layer 15, therefore, the silicon single-crystalline layer 4 readily causes crystal defects. Such crystal defects in the silicon single-crystalline layer 4 may influence the crystal state of the silicon oxide layer 5 formed subsequently to the silicon single-crystalline layer 4, to generate a leakage current between the elements again.
Further, the porous silicon layer 15 merely serving as a stopper must be removed after the bonding step. The porous silicon layer 15 cannot be employed as a layer (hereinafter referred to as a device forming layer) for forming the device on the surface of the SOI substrate due to the inferior crystal state thereof. However, this is inefficient in consideration of effective use of the raw material.
According to a first aspect of the present invention, a method of manufacturing an SOI substrate comprises steps (a) to (f) of (a) forming a silicon germanium single-crystalline layer on a main surface of a bond wafer consisting of a single crystal of silicon, (b) forming a silicon single-crystalline layer on a surface of the silicon germanium single-crystalline layer, (c) oxidizing a surface of the silicon single-crystalline layer, (d) bonding a base wafer consisting of a single crystal of silicon to the oxidized surface of the silicon single-crystalline layer, (e) heating the bond wafer and the base wafer for reinforcing the degree of adhesion therebetween, and (f) removing the bond wafer.
In the method of manufacturing an SOI substrate according to the first aspect, the silicon germanium single-crystalline layer exhibiting small irregularity in its crystal state and having selectivity for the single crystal of silicon forming the bond wafer hardly causes crystal defects in the silicon single-crystalline layer and reliably enables removal of the bond wafer. Further, the silicon germanium single-crystalline layer can be employed as a device forming layer on the SOI substrate.
According to a second aspect of the present invention, the method of manufacturing an SOI substrate according to the first aspect further comprises a step (g) of reducing the thickness of the silicon germanium single-crystalline layer to a prescribed value subsequently to the step (f).
In the method of manufacturing an SOI substrate according to the second aspect, the thickness of the silicon germanium single-crystalline layer can be set to a level suitable for serving as a device forming layer. Alternatively, the silicon germanium single-crystalline layer can be completely removed for manufacturing a general SOI substrate comprising only a buried silicon oxide layer and a silicon layer on the base wafer.
According to a third aspect of the present invention, a part of the bond wafer in contact with the silicon germanium single-crystalline layer is removed by chemical mechanical polishing or wet etching in the step (f), and the silicon germanium single-crystalline layer is removed by wet etching in the step (g).
In the method of manufacturing an SOI substrate according to the third aspect, not plasma etching but chemical mechanical polishing or wet etching is employed for finishing removal of the bond wafer, whereby the silicon germanium single-crystalline layer has a small possibility of causing crystal defects. Further, the silicon germanium single-crystalline layer is removed by wet etching, whereby the silicon germanium single-crystalline layer and the silicon single-crystalline layer have a small possibility of causing crystal defects.
According to a fourth aspect of the present invention, the method of manufacturing an SOI substrate according to the first aspect further comprises steps (h), (k) and (i) of (h) forming a mask layer on the silicon-germanium single-crystalline layer after the step (f), (k) patterning the mask layer through photolithography, and (i) removing a part of the silicon-germanium single-crystalline layer not covered with the mask layer by employing the patterned mask layer as a mask.
In the method of manufacturing an SOI substrate according to the fourth aspect of the present invention, the silicon germanium single-crystalline layer can be subjected to arbitrary patterning for serving as a device forming layer.
According to a fifth aspect of the present invention, the method of manufacturing an SOI substrate according to the fourth aspect further comprises a step (j) of oxidizing the part of the silicon-germanium single-crystalline layer not covered with the mask layer after the step (k) in advance of the step (i) for removing the oxidized part of the silicon-germanium single-crystalline layer by wet etching in the step (i).
In the method of manufacturing an SOI substrate according to the fifth aspect, the oxidized part of the silicon germanium single-crystalline layer is removed not by plasma etching but by wet etching when the silicon germanium single-crystalline layer is subjected to arbitrary patterning for serving as a device forming layer, whereby the silicon germanium single-crystalline layer and the silicon single-crystalline layer have a small possibility of causing crystal defects.
According to a sixth aspect of the present invention, the mask layer has a multilayer structure obtained by forming a silicon nitride film on the upper surface of a silicon oxide film, and a photoresist film is formed on a surface of the multilayer structure and the photoresist film is patterned through photolithography for patterning the mask layer by removing a part of the multilayer structure not covered with the photoresist film by employing the photoresist film as a mask in the step (k).
In the method of manufacturing an SOI substrate according to the sixth aspect, the silicon nitride film serves as an anti-oxidation film in a later eleventh step, while the silicon oxide film prevents nitrogen contained in the silicon nitride film from permeating into the surface of the wafer.
According to a seventh aspect of the present invention, the silicon germanium single-crystalline layer is employed as a device forming layer.
In the method of manufacturing an SOI substrate according to the seventh aspect, the crystal state of the silicon germanium single-crystalline layer is so excellent that a device having excellent break-down voltage can be manufactured. Further, mobility of holes in the silicon germanium single-crystalline layer is higher than that in silicon and hence the operating speed of the device can be increased.
According to an eighth aspect of the present invention, the device forming layer is employed as a channel and a source/drain region of a MOSFET.
In the method of manufacturing an SOI substrate according to the eighth aspect, the crystal state of the silicon germanium single-crystalline layer is so excellent that a MOSFET having excellent break-down voltage can be manufactured. Further, mobility of holes is higher than that in silicon and hence the operating speed of a P-channel MOSFET can be increased.
According to a ninth aspect of the present invention, a part of the silicon single-crystalline layer exposed by removal of the silicon germanium single-crystalline layer is employed as a channel and a source/drain region as to an N-channel MOSFET included in the MOSFET.
In the method of manufacturing an SOI substrate according to the ninth aspect, the silicon single-crystalline layer is employed as the channel, whereby the operating speed of the N-channel MOSFET is higher than that in the case of employing the silicon germanium single-crystalline layer as the channel.
According to a tenth aspect of the present invention, the device forming layer is an infrared detection part of an infrared detector.
In the method of manufacturing an SOI substrate according to the tenth aspect, the crystal state of the silicon germanium single-crystalline layer is so excellent that an infrared detector having excellent detection sensitivity can be manufactured.
According to an eleventh aspect of the present invention, another silicon single-crystalline layer is further formed on the upper surface of the silicon germanium single-crystalline layer, the device forming layer is a base layer of a heterojunction bipolar transistor, and one of the silicon single-crystalline layer formed on the upper surface of the silicon germanium single-crystalline layer and the silicon single-crystalline layer present on the lower surface of the silicon germanium single-crystalline layer is a collector layer of the heterojunction bipolar transistor, and the other is an emitter layer of the heterojunction bipolar transistor.
In the method of manufacturing an SOI substrate according to the eleventh aspect, the crystal state of the silicon germanium single-crystalline layer is so excellent that a heterojunction having a small number of interfacial states can be formed. Further, the crystal state of the silicon germanium single-crystalline layer is so excellent that a heterojunction bipolar transistor having excellent break-down voltage can be manufactured. In addition, mobility of holes in the silicon germanium single-crystalline layer is higher than that in silicon, and hence the operating speed of a PNP heterojunction bipolar transistor can be increased.
According to a twelfth aspect of the present invention, a method of manufacturing an SOI substrate comprises steps (a) to (c) of (a) forming a mask layer on a surface of an SOI substrate comprising a base wafer consisting of a single crystal of silicon, a silicon oxide film formed on a surface of the base wafer, a silicon single-crystalline layer formed on a surface of the silicon oxide film and a silicon germanium single-crystalline layer formed on a surface of the silicon single-crystalline layer, (b) patterning the mask layer through photolithography, and (c) removing a part of the silicon germanium single-crystalline layer not covered with the mask layer by employing the patterned mask layer as a mask.
In the method of manufacturing an SOI substrate according to the twelfth aspect, an effect similar to that in the method of manufacturing an SOI substrate according to the fourth aspect can be attained.
According to a thirteenth aspect of the present invention, the method of manufacturing an SOI substrate according to the twelfth aspect further comprises a step (d) of oxidizing the part of the silicon germanium single-crystalline layer not covered with the mask layer after the step (b) in advance of the step (c) for removing the oxidized part of the silicon germanium single-crystalline layer by wet etching in the step (c).
In the method of manufacturing an SOI substrate according to the thirteenth aspect, an effect similar to that in the method of manufacturing an SOI substrate according to the fifth aspect can be attained.
According to a fourteenth aspect of the present invention, the mask layer has a multilayer structure obtained by forming a silicon nitride film on the upper surface of a silicon oxide film, a photoresist film is formed on a surface of the multilayer structure, and the photoresist film is patterned through photolithography for patterning the mask layer by removing a part of the multilayer structure not covered with the photoresist film by employing the photoresist film as a mask in the step (b).
In the method of manufacturing an SOI substrate according to the fourteenth aspect, an effect similar to that in the method of manufacturing an SOI substrate according to the sixth aspect can be attained.
According to a fifteenth aspect of the present invention, the silicon germanium single-crystalline layer is employed as a device forming layer.
In the method of manufacturing an SOI substrate according to the fifteenth aspect, the crystal state of the silicon germanium single-crystalline layer is so excellent that a device having excellent break-down voltage can be manufactured. Further, mobility of holes in the silicon germanium single-crystalline layer is higher than that in silicon and hence the operating speed of the device can be improved.
According to a sixteenth aspect of the present invention, the device forming layer is employed as a channel and a source/drain region of a MOSFET.
In the method of manufacturing an SOI substrate according to the sixteenth aspect, the crystal state of the silicon germanium single-crystalline layer is so excellent that a MOSFET having excellent break-down voltage can be manufactured. Further, mobility of holes is higher than that in silicon and hence the operating speed of a P-channel MOSFET can be increased.
According to a seventeenth aspect of the present invention, a part of the silicon single-crystalline layer exposed by removal of the silicon germanium single-crystalline layer is employed as a channel and a source/drain region as to an N-channel MOSFET included in the MOSFET.
In the method of manufacturing an SOI substrate according to the seventeenth aspect, the silicon single-crystalline layer is employed as the channel, whereby the operating speed of the N-channel MOSFET is higher than that in the case of employing the silicon germanium single-crystalline layer as the channel.
According to an eighteenth aspect of the present invention, the device forming layer is an infrared detection part of an infrared detector.
In the method of manufacturing an SOI substrate according to the eighteenth aspect, the crystal state of the silicon germanium single-crystalline layer is so excellent that an infrared detector having excellent detection sensitivity can be manufactured.
According to a nineteenth aspect of the present invention, another silicon single-crystalline layer is further formed on the upper surface of the silicon germanium single-crystalline layer, the device forming layer is a base layer of a heterojunction bipolar transistor, and one of the silicon single-crystalline layer formed on the upper surface of the silicon germanium single-crystalline layer and the silicon single-crystalline layer present on the lower surface of the silicon germanium single-crystalline layer is a collector layer of the heterojunction bipolar transistor, and the other is an emitter layer of the heterojunction bipolar transistor.
In the method of manufacturing an SOI substrate according to the nineteenth aspect, the crystal state of the silicon germanium single-crystalline layer is so excellent that a heterojunction having a small number of interfacial states can be formed. Further, the crystal state of the silicon germanium single-crystalline layer is so excellent that a heterojunction bipolar transistor having excellent break-down voltage can be manufactured. In addition, mobility of holes in the silicon germanium single-crystalline layer is higher than that in silicon and hence the operating speed of a PNP heterojunction bipolar transistor can be increased.
An object of the present invention is to implement a method of manufacturing an SOI substrate by bonding, which can employ a layer exhibiting small irregularity in its crystal state as a stopper having selectivity for single-crystalline silicon and effectively utilize the stopper as a device forming layer.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.